In recent years, the electrophotography has been applied to copying machines as well as various printers since they can give images with high qualities without delay. As an photoreceptor which plays an important role in the electrophotography, the photoreceptor comprising an inorganic photoconductive material such as selenium, arsenic-selenium alloy, cadmium sulfide, zinc oxide and the like has been used. More recently, the photoreceptor comprising an organic photoconductive material was proposed. The latter has the advantages which is not a pollutant and which has a film-formability and a shapability.
As one of the organic photoreceptors, the so-called "laminated-type photoreceptor" in which a charge generation layer and a charge transfer layer are successively laminated was developed. The laminated-type photoreceptor is increasingly interested in and is expected to be widely used in the near future because it has the following advantages:
(1) the photoreceptor having high sensitivity can be obtained by suitably selecting and combining the charge generation material and the charge transfer material; PA0 (2) the photoreceptor having high safety can be obtained because the charge generation material and the charge transfer material can be selected from a wide range of the materials; and PA0 (3) the photoreceptor can be prepared by simple coating and thus it can be prepared with low costs.
However, the prior laminated-type photoreceptors cause the electric problems such as the lowering of the charged potential, the accumulation of the residual potential and the change in the sensitivity by their repeated use. The problem as to the accumulation of the residual potential is especially serious because if the residual potential is accumulated, much copies could not be obtained. Such an accumulation of the residual potential is considered to arise from some causes, among which impurities present in the charge transfer layer are important. That is, impurities trap carriers so as to produce unmovable space charges which remain as the residual charges in the charge transfer layer. The increase of the thickness of the charge transfer layer is effective for reducing the effects of the thinning down of the charge transfer layer caused by the abrasion such as blade cleaning on the electric properties and increasing the sensitivity of the photoreceptor, but it is accompanied with the increase of the amounts of impurities so that the accumulation of the residual potential makes more remarkable.
For preventing the accumulation of the residual potential caused by the impurities present in the charge transfer layer, an addition of an electron attractive compound in the charge transfer layer is attempted. Generally the addition of the electron attractive compound in the electron donative compound forms a charge transfer complex and as the result a new absorption band appears in the long wavelength region. Thus, it is considered that the accumulation of the residual potential could be prevented by irradiating the light having the wavelength which corresponds to the absorption band of the charge transfer complex to the photoreceptor so as to form a few movable carriers in the charge transfer layer, which carriers neutralize the unmovable space charges. The known electron attractive compounds are not satisfactory for preventing the accumulation of the residual potential. Further they have another disadvantages such as the increase in the dark decay, the lowering in the surface voltage and the lowering in the sensitivity by the repeated use.
The present inventors have been investigated the electron attractive compounds which can prevent the accumulation of the residual potential without affecting the other electric properties and now they found that specific sulfonate ester compounds satisfy the above requirements.